FIG. 1 shows a typical “sandwich” assay for identifying and quantifying an antigen. This is called a sandwich assay because it uses 2 or more antibodies to sandwich the antigen. In the typical sandwich assay a primary antibody is bound to a support substrate. This primary antibody is used as a capture antibody to capture a specific antigen in solution. A second antibody that attaches to the same antigen as the primary antibody is then added. This second antibody includes a tag of some sort for identifying the presence of the second antibody attached to the antigen. This tag may include, for example, a complex that fluoresces when exposed to laser light. This fluorescent signal can then be detected by a separate light detector.
One Example of a sandwich assay is an Enzyme-Linked Immunosorbent Assay (ELISA or EIA for short). The ELISA assay utilizes two antibodies, one of which is specific to the antigen and the other of which is coupled to an enzyme. This second antibody gives the assay its “enzyme-linked” name. The enzyme promotes a reaction involving an enzyme converted substrate, which produces a signal that can be proportional to the amount of the antigen that is present.
One drawback of the sandwich assay technique is the number of steps that are typically required to carry out the technique. For example, at least the following steps are typically performed: 1) Prepare a surface plate to which a known quantity of antibody is bound; 2) Apply the antigen-containing sample to the plate; 3) Wash the plate, so that unbound antigen is removed; 4) Apply the enzyme-linked antibodies which are also specific to the antigen; 5) Wash the plate, so that unbound enzyme-linked antibodies are removed; 6) Apply a chemical which is converted by the enzyme into a chemical that can generate fluorescent or color signal; and 7) View the result: if it fluoresces, then the sample contained antigen.
In addition to the number of steps, several separate specialized compositions and devices are used in the sandwhich assay technique. For example, two different antibodies are used and a separate detector for detecting the fluorescence is used.